Everything I will say in this post is made up. Please correct all errors. Low pH of wort will produce a layer of corrosion. Acid in wort is mainly phosphoric, so corrosion would be copper phosphate? Once underlying copper is protected it should not be a factor. Wort is only in container long enough to inoculate itself.

I haven't used a coolship, but I have had success with capturing wild yeast. I followed the bootleg biology's instructions. I did the capturing in the fall. I would think about now is fine too, you want the cool evening and over night temps.